1. Technical Field of the Invention
The field of the invention is that of viewing devices intended to be mounted in vehicles and making it possible to display all or part of the information displayed on the exterior. The favoured field of applications is aeronautics where these viewing devices are mounted on aircraft instrument panels. However, this type of device can also be mounted on road or railway vehicles.
2. Description of the Prior Art
In a conventional manner, in an aircraft cockpit, when it is desired to display information on the exterior, a specific viewing device is dedicated to this function. This device, generally called “Head-Up” or “HUD”, the acronym standing for “Head-Up Display”, is totally separate from the so-called “Head-Down” viewing devices which currently consist of liquid-crystal flat screens. Generally, an HUD chiefly comprises an image source, a collimation optic and an optical mixer or “combiner”. The collimated image is superimposed on the exterior by means of the optical mixer. Such an assembly combining “Head-Down” viewing units and “Head-Up” viewing units has several drawbacks. The first is of an ergonomic nature. The image of the screens on the instrument panel is at finite distance whereas the image of the collimated viewing units is at large distance. It is therefore very difficult to ensure continuity in the two images and, moreover, the observer must accommodate when he switches from one representation to another. However, the accommodation time is not negligible, having regard to the high speed of the carrier. The second drawback is of an economic nature. An “HUD” is an expensive optical device subject to specific production, installation and harmonization constraints.